This invention is concerned with improving the octane rating of gasoline produced by fluid catalytic cracking of gas oil feedstock with a zeolitic cracking catalyst having a low sodium content.
Fluid catalytic cracking of hydrocarbon charge to produce gasoline involves cyclic contact of the charge at cracking temperature with a particle form solid cracking catalyst, whereby components of the charge are converted by cracking to lower boiling hydrocarbons including a gasoline fraction with concurrent deposition on the catalyst of an inactivating carbonaceous contaminant. Gasoline is recovered from the products of conversion. Optionally hydrocarbon is stripped by steam from the catalyst particles before activity of the contaminated catalyst is restored by burning the carbonaceous deposit. Catalyst so regenerated is contacted with additional hydrocarbon charge, whereby the catalyst declines in regenerated activity over repeated cycles of charge contact and regeneration. The average activity of the catalyst inventory is maintained at substantially constant equilibrium values by replacing a portion of the catalyst inventory with fresh catalyst of activity above equilibrium values.
When crystalline synthetic zeolite cracking catalysts were introduced to the petroleum industry, improved yields and product slates were realized. These catalysts are composites of crystals of synthetic aluminosilicates disseminated in a porous inorganic oxide matrix. Reference is made to U.S. Pat. Nos. 3,140,249 and 3,932,268 as illustrative examples. In particular, higher gasoline yields were achieved with these highly active catalysts. Modification of cracking equipment and processing parameters to optimize the usefulness of the new zeolitic catalyst followed. While high gasoline yields were achieved at high conversion rates with the synthetic crystalline zeolite catalysts, it was found that gasoline from the fluid cracking units generally had significantly lower octane numbers than gasoline from crackers using catalyst of the type made obsolete by the availability of the more active zeolitic catalysts. The view has been held that the hydrogen transfer capability of the zeolitic catalysts was responsible for the relatively low yield of olefins and that this change in gasoline composition resulted in loss in octane. Some improvement in octane has been achieved by operating the cracker at increased reactor temperature. However, due to recent EPA rulings requiring phase out of lead to improve octane there is increasing pressure on refineries to increase the octane number of FCC gasoline to levels beyond that readily achieved by variation in operation of the cracking unit.
It has been proposed to improve the octane rating of gasoline from an FCC unit by emloying an ultrastable zeolite promoter in the cracking catalyst composition, the zeolite preferably being free of rare earth metal. Reference is made to British Pat. No. 2,022,439. The term "ultrastable" as used in this patent refers to a family of synthetic crystalline aluminosilicate zeolite having a low sodium content and prepared in accordance with the teaching of U.S. Pat. No. 3,293,192 discussed below. The British Patent teaches that the weight percentage sodium oxide (Na.sub.2 O) in the total catalyst charge to the FCC unit divided by the weight of the zeolite in the catalyst should be equal or less than 0.013. A generally similar disclosure appears in U.S. Pat. No. 3,944,800. The U.S. Pat. No. 3,944,800 discloses that increased yields of more olefinic products are realized when operating FCC units under typical cracking conditions by using a zeolite having a low sodium content and produced by ammonium exchange of a zeolite of the Y type (U.S. Pat. No. 3,130,007), followed by calcination and re-exchange with ammonium. The initial ion-exchange with ammonium salt is carried out at a controlled acidic pH. The resultng ultrastable H-Y zeolite is prepared separately and then combined with the matrix. The patent focuses on the sodium level of the zeolite per se which is in contrast to concern with the sodium level of the catalyst composition.
The effect of the species of exchangeable cations on the catalytic cracking activity is described in a paper "Ion-Exchanged Ultrastable Y Zeolites. 3. Gas Oil Cracking over Rare Earth-Exchanged Ultrastable Y Zeolites" by Julius Scherzer and Ronald E. Ritter, W. R. Grace, Ind. Eng. Chem. Prod. Res. Dev.; 17; 3, 219 (1978). Rare earth exchanged ultrastable zeolites (Re H-USY) are shown to be more active for cracking than are H-ultrastable zeolites (H-USY). However, even the Re-H-USY is less active than typical Re-H-Y zeolite. Re-H-Y is shown to have a higher concentration of Bronsted acid sites than do the H-USY zeolites. The authors suggest that the lower density of acid sites in USY zeolites the rate of conversion of olefins into paraffins and of aromatics into condensed polycyclics (coke), thus allowing the olefins and aromatics to diffuse out of the zeolite and to desorb. Exchange of rare earth into H-USY zeolites tends to increase the rate of these hydrogen-transfer reactions, resulting in more coke and higher conversions. The authors do not discuss the effects of sodium exchange or contamination which would be expected to decrease hydrogen-transfer reactions and lower conversion.
A correlation between acid site density and the composition of FCC gasoline appears in the following publication: "Formation of High Octane Gasoline by Zeolite Cracking Catalysts" by J. S. Magee and R. E. Ritter, W. R. Grace, Paper Presented at ACS Meeting, Sept. 10-15, 1978, Miami Beach, Florida. This paper discusses various process conditions, feedstock and catalyst effects on octane. Y zeolites exchanged with either hydrogen or rare earth are claimed to yield similar octane gasoline at constant severity and conversion level. The authors allege that a recently introduced cracking catalyst that is free from rare earth produces higher octane gasoline than does a commercial REY containing catalyst. Increased aromatic and olefin content of the gasoline produced by that catalyst is claimed. The authors postulate that a combination of reduced acid site density and a change (increase) in the ratio of Lewis/Bronsted acid sites may be causing the observed difference in product quality. No recognition is expressed in this paper of the influence of alkali content of either the fresh or equilibrium catalyst on octane.
Low sodium content zeolites and low sodium content zeolitic cracking catalysts are extensively described in the literature. U.S. Pat. No. 3,293,192 (supra) is an early example of a description of a low sodium zeolite. However this patent is concerned with the production of an ultrastable zeolite but not the use of such zeolite as a component of a cracking catalyst. This is also true of U.S. Pat. No. 3,449,070 which discloses alternative processing to provide an ultrastable zeolite. U.S. Pat. No. Re. 28,629 describes a process for cation exchanging zeolites to produce a low sodium content zeolite product which involved ion-exchanging sodium in a synthetic crystalline zeolite with a solution of a salt of at least one desired metal cation to an alkali content about 3 to 4 weight percent, washing, drying and heating to 400.degree.-1500.degree. F. to redistribute locked-in cations, ion-exchanging to further reduce alkali metal content and drying.
A similar disclosure appears in U.S. Pat. No. 4,058,484 which describes the preparation of stabilized HY zeolites having a Na.sub.2 O level below 1.5% and a crystallinity substantially the same as NaY. However this stabilized HY zeolite does not have a reduced cell size such as the reduced cell size which characterizes the ultrastable zeolites described in U.S. Pat. Nos. 3,293,192 and 3,449,070. As described in U.S. Pat. No. 4,058,484 a sodium zeolite Y is ion-exchanged with a ammonium salt solution at a pH of 3-4, heated under relatively mild conditions (300.degree.-400.degree. F.) and washed. The lower sodium level is alleged to improve hydrothermal stability but also to provide increased resistance to metal poisoning. U.S. Pat. No. 4,085,069 is directed to a method of producing a cracking catalyst containing 10-30% of the stabilized low sodium content faujasite of U.S. Pat. No. 4,058,484 (supra). As described in U.S. Pat. No. 4,085,069 such zeolite is composited with 20-70% clay and 10-30% peptized alumina. Again improved stability is alleged but this is the only benefit of maintaining low sodium in the overall catalyst composition. U.S. Pat. No. 4,100,108 alleges a synergistic effect on cracking activity by employing a mixture of two faujasite type zeolites, one containing 21/2-5% Na.sub.2 O and the other less than 21/2% Na.sub.2 O in a matrix of alumina and clay. The low sodium zeolite is shown to provide increased activity after a high temperature (1550.degree. F.) laboratory steaming whereas the higher sodium rare earth zeolite provides a higher activity after a 1450.degree. F. steaming. In U.S. Pat. No. 4,100,108 there is no expression of appreciation of the effect of sodium content of the catalyst on octane.
While it is known that high octane gasoline is obtainable, at least on laboratory scale testing, by utilizing cracking catalysts containing ultrastable zeolites which, by their nature are low in sodium content, in practice this knowledge has not led to a commercially significant advance in the operation of catalytic crackers. It has been observed that low sodium catalyst products which, based on laboratory evaluations, promise to enhance octane when employed in commercial FCC units have produced disappointing and perplexing results when operating with equilibrium catalyst.